DESCRIPTION

The rcmd() function is used by the superuser to execute a command on a
remote machine using an authentication scheme based on reserved port
numbers. If the calling process is not setuid, the RSH environment vari-
able is set, and inport is "shell/tcp", rcmdsh(3) is called instead with
the value of RSH. Alternately, if the user is not the superuser, rcmd()
will invoke rcmdsh(3) to run the command via rsh(1). While rcmd() can
handle IPv4 cases only, the rcmd_af() function can handle other cases as
well.
The rresvport() and rresvport_af() functions return a descriptor to a
socket with an address in the privileged port space. The iruserok() and
ruserok() functions are used by servers to authenticate clients request-
ing service with rcmd(). All four functions are present in the same file
and are used by the rshd(8) server (among others). iruserok_sa() is an
address family independent variant of iruserok().
The rcmd() function looks up the host *ahost using gethostbyname(3), re-
turning -1 if the host does not exist. Otherwise *ahost is set to the
standard name of the host and a connection is established to a server
residing at the well-known Internet port inport. If the user is not the
superuser, the only valid port is "shell/tcp" (usually port 514).
If the connection succeeds, a socket in the Internet domain of type
SOCK_STREAM is returned to the caller, and given to the remote command as
stdin and stdout. If fd2p is non-zero, then an auxiliary channel to a
control process will be set up, and a descriptor for it will be placed in
*fd2p. The control process will return diagnostic output from the command
(unit 2) on this channel, and will also accept bytes on this channel as
being UNIX signal numbers, to be forwarded to the process group of the
command. If fd2p is NULL, then the standard error (unit 2 of the remote
command) will be made the same as the standard output and no provision is
made for sending arbitrary signals to the remote process, although you
may be able to get its attention by using out-of-band data. Note that if
the user is not the superuser, fd2p must be NULL.
rcmd_af() takes address family in the last argument. If the last argument
is PF_UNSPEC, interpretation of *ahost will obey the underlying address
resolution like DNS.
The protocol is described in detail in rshd(8).
The rresvport() and rresvport_af() functions are used to obtain a socket
with a privileged address bound to it. This socket is suitable for use by
rcmd() and several other functions. Privileged Internet ports are those
in the range 0 to IPPORT_RESERVED - 1, which happens to be 1023. Only the
superuser is allowed to bind an address of this sort to a socket.
rresvport() and rresvport_af() need to be seeded with a port number; if
that port is not available these functions will find another.
The iruserok() and ruserok() functions take a remote host's IP address or
name, respectively, two user names and a flag indicating whether the lo-
cal user's name is that of the superuser. Then, if the user is not the
superuser, it checks the /etc/hosts.equiv file. If that lookup is not
done, or is unsuccessful, the .rhosts in the local user's home directory
is checked to see if the request for service is allowed.
If this file does not exist, is not a regular file, is owned by anyone
other than the user or the superuser, or is writeable by anyone other
than the owner, the check automatically fails. Zero is returned if the
machine name is listed in the hosts.equiv file, or the host and remote
user name are found in the .rhosts file; otherwise iruserok() and
ruserok() return -1. If the local domain (as obtained from
gethostname(3)) is the same as the remote domain, only the machine name
need be specified.
If the IP address of the remote host is known, iruserok() should be used
in preference to ruserok(), as it does not require trusting the DNS
server for the remote host's domain.
While iruserok() can handle IPv4 addresses only, iruserok_sa() and
ruserok() can handle other address families as well, like IPv6. The first
argument of iruserok_sa() is typed as void * to avoid dependency between
<unistd.h> and <sys/socket.h>.

DIAGNOSTICS

The rcmd() function returns a valid socket descriptor on success. It re-
turns -1 on error and prints a diagnostic message on the standard error.
The rresvport() and rresvport_af() functions return a valid, bound socket
descriptor on success. It returns -1 on error with the global value errno
set according to the reason for failure. The error code EAGAIN is over-
loaded to mean "all network ports in use".